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The bitter harvest of self-inflicted wounds
Formation of bulky fruits relies on good transport mechanisms from source to sink. However, there is evidence from some species that these transport mechanisms may not be as efficient as we might suppose. Apples, for example, may be prone to a range of conditions that result from calcium deficiency, e.g. bitter pit. This implies that the transport of calcium is hindered in some way. As is pointed out by Drazeta and colleagues (Palmerston North and Havelock North, New Zealand, pp. 275-282) calcium is a xylem-transported element and it is well-known that xylem becomes dysfunctional during fruit development. They therefore studied this progressive loss of xylem function to determine whether there are differences between apple varieties that differ in their proneness to bitter pit. The varieties selected were ‘Braeburn’ and ‘Granny Smith’, the former being the more susceptible. Previous studies had already shown a decline in calcium accumulation that was correlated with decline in xylem function. In this study, the authors used transport of dye to study xylem function and showed that xylem dysfunction worked its way back from within the fruit towards the stalk and that the primary bundles were particularly affected. Especially interesting was the finding that in ‘Braeburn’, the susceptible variety, loss of xylem function occurred significantly earlier than in ‘Granny Smith’. Indeed in the latter variety there was still some xylem function in ripe fruit. But why does the xylem become dysfunctional? Microscopic examination of damaged xylem and of the parenchyma of the growing fruit leads the authors to suggest that the rapid growth of the fruit causes stress-deformation and finally breakage in the xylem vessels, which then collapse. There still remains the puzzle of why the phloem in the same bundles remains functional. Nevertheless, these data demonstrate clear correlations between fruit growth, xylem damage, calcium transport and susceptibility to bitter pit.
Professor J. A. BryantUniversity of Exeter, UK
j.a.bryant{at}exeter.ac.uk
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